1. Field of the Invention
The present invention relates to a coordinate input device and display-integrated type coordinate input device for use in computers, word processors, for example.
2. Related Art
As an input device for effecting an input into a computer, a word processor or the like, there is a well known tablet capable of inputting figures and letters, giving instructions for command execution and effecting other functions under instructions from a pen, a finger or the like. As an input device developed from the above tablet, a display-integrated type tablet device that commonly uses the electrodes and drive circuits of an LCD (Liquid Crystal Display) panel for the achievement of an inexpensive structure is disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 5-53726.
This display-integrated type tablet device has a construction as shown in FIG. 35, where a switchover circuit 2 selects a detection control circuit 3 in a coordinate detection period under the control of a control circuit 1. Then, on the basis of signals from the detection control circuit 3, a segment electrode drive circuit 4 generates a segment electrode scanning signal to sequentially scan the segment electrodes X of an LCD panel 5. Subsequently, a common electrode drive circuit 6 generates a common electrode scanning signal to sequentially scan the common electrodes Y. An x-coordinate detection circuit 7 and a y-coordinate detection circuit 8 detect the coordinates of the tip of an electronic pen 9 on the basis of an induction voltage induced at the tip electrode of the electronic pen 9. On the other hand, the switchover circuit 2 selects a display control circuit 10 during a display period and controls the segment electrode drive circuit 4 and the common electrode drive circuit 6, thereby displaying an image on the LCD panel 5.
There is further disclosed a transparent digitizer that is concurrently used as a touch panel for detecting the position of an operator's finger or a codeless pen in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 8-179871. This touch panel and transparent digitizer has a construction as shown in FIG. 36. When it operates as a digitizer, an AC signal oscillated from an oscillation pen 11 is received as a voltage of the electrode X of a first tablet 12 and a voltage of the electrode Y of a second tablet 13, the voltages being induced by electrostatic capacitive coupling. Then, a signal level obtained every adjacent electrodes X.sub.n and X.sub.n+1 or adjacent electrodes y.sub.m and Y.sub.m+1 is subjected to a balance processing in a balance input circuit 14, and only necessary frequency signal is taken out through a band-pass filter 15. A control circuit 16 detects the position of the oscillation pen 11 based on the above signal.
When operating as a touch panel, the control circuit 16 controls analog switches 17 and 18 to sequentially apply a signal from a signal oscillator 19 to the electrodes X and the electrodes Y. Then, by detecting a change in electrostatic capacitance of the electrodes that are facing each other, the position that the finger touches is detected.
A touch panel for detecting the position of the operator's finger is disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. SHO 60-123927. In this touch panel, one electrode that is sequentially selected out of a plurality of electrodes arranged parallel to one another is connected to a signal detection circuit, while the other electrodes are connected to a signal source for generating an AC signal. Then, a signal induced at the electrode by the electrostatic capacitance between the operator's finger and the electrode is detected by the above signal detection circuit, and the position that the finger touches is detected based on this detection signal. In this case, if the width of the electrode located on the upper side is wide in detecting the signal induced at the electrode positioned on the lower side, the upper electrode disadvantageously functions as an electrostatic shield, so that sufficient signal detection cannot be performed. Therefore, the width of the electrode located on the upper side is narrower than the width of the electrode located on the lower side.
However, the aforementioned prior art display-integrated type tablet device, touch panel and transparent digitizer and touch panel have.
In the above display-integrated type tablet device, an infinitesimal induction voltage induced at the tip electrode of the electronic pen 9 is amplified and transmitted to the x-coordinate detection circuit 7 and the y-coordinate detection circuit 8, and both the detection circuits 7 and 8 detect the x-coordinate value and the y-coordinate value. Therefore, a cable arranged between the electronic pen 9 and both the detection circuits 7 and 8 is required. The cable causes a sense of incongruity in the operator, and its pull force significantly reduces the operability of the electronic pen 9. Furthermore, when the above cable is located in a position on the LCD panel 5 at which the operator is about to look, the screen is not easy to view, resulting in a bad operability. Furthermore, due to the above construction, pointing cannot be executed by finger touch.
In contrast to the above, the touch panel and transparent digitizer requires no cable between the oscillation pen 11 and the control circuit 16, achieving a codeless pen. However, the oscillation pen 11 must be mounted with an oscillation circuit and a battery for driving this oscillation circuit, which increase the weight and significantly reduce the operability. Additionally, the above battery must be periodically replaced, which is troublesome.
Furthermore, when mounting this touch panel and transparent digitizer on the display device for the provision of a display-integrated type tablet, there is an increase in number of components for the reason that the control circuit of the digitizer and the control circuit of the display device are independent of each other. Moreover, an adjusting process is necessary for the coincidence of a detection point with a display point in an assembling process, as a consequence of which the number of processes increases to cause a cost increase. Additionally, the touch panel and transparent digitizer is provided with the electrodes X and Y formed of an ITO (Indium Tin Oxide) film for example, part of light from the above display device is reflected and absorbed by the electrodes X and Y, causing a deterioration in visibility. Furthermore, for the mounting of the touch panel and transparent digitizer on the display device, the thickness of the device increases to significantly hinder the achievement of a thin lightweight structure.
The common use of the electrodes X and Y of the above touch panel and transparent digitizer as the electrodes of the display device for the formation of a display-integrated type tablet is very difficult for the following reasons.
That is, generally in a display device employing a liquid crystal display, an EL (electroluminescence) display, a plasma display or the like, electrodes that are perpendicular to each other are arranged on two substrates separated at a specified interval. As an example, a cross-section of a duty ratio type LCD device of STN (Super Twisted Nematic) type or the like is shown in FIG. 37.
Although schematically shown in FIG. 37, practically an upper glass substrate 21 and a lower glass substrate 22 have a thickness of about 0.7 mm, and an upper polarizing plate 23 and a lower polarizing plate 24 have a thickness of about 0.1 mm. In contrast to this, a liquid crystal spacer 25 has a thickness of 7 .mu.m, so that a distance from the electrodes 26 and 27 to a finger 28 is about 100 times a distance from a segment electrode 26 to a common electrode 27.
Furthermore, a transparent protection panel for protecting an LCD device is sometimes mounted on this LCD device with interposition of a specified gap. This protection panel is provided for preventing the possible occurrence of scratches on the surface of the LCD device, deterioration of the LCD device due to direct application of a pressure to it and break of the glass substrates 21 and 22, and it is mounted with an air gap provided taking the warp of the protection panel occurring when it is pressed into consideration. In general, the above protection panel is provided by transparent acrylic or the like having a thickness of 1 mm, and the air gap is set to about 2 mm. When this protection panel is mounted, the distance from the electrodes 26 and 27 to the finger 28 is about 540 times the distance from the segment electrode 26 to the common electrode 27.
There is an active addressing type LCD device such as the TFT (Thin Film Transistor) type having a high display quality. In the case of this LCD device, source electrodes and gate electrodes are deposited on an identical glass substrate. Therefore, the distance from the electrodes to the finger 28 is about 1000 times the distance from the source electrode to the gate electrode.
As described above, in each of the aforementioned LCD devices, the distance from the electrodes to the finger is about 100 to 1000 times the distance between the electrodes that are perpendicular to each other. Therefore, the coupling capacitance between the electrodes that are perpendicular to each other is much greater than the coupling capacitance between the finger and the electrodes, and an electric line of force to be shielded by a shielding object such as the finger among the electric lines of force coming from one electrode is very small, so that it is in practice very difficult to detect a change in signal between adjacent electrodes.
In the touch panel disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. SHO 60-123927, one electrode to be sequentially selected out of the plurality of electrodes arranged parallel to each other is connected to the signal detection circuit, and the other electrodes are connected to the signal source for generating an AC signal, by which the signal induced at the electrode due to the electrostatic coupling capacitance between the operator's finger and the electrodes is detected by the signal detection circuit. This arrangement obviates the need for using a pen mounted with a cable, an oscillation circuit and a battery.
However, the drive circuit of the touch panel is normally formed of a multi-layer substrate, and an electrostatic capacitance exists between wiring lines. An electrostatic coupling capacitance also exists between the electrodes. If the wiring lines and electrodes to which a DC voltage of a control signal or the like is applied exist in such a state, then the phase of the AC signal is rotated by the electrostatic coupling capacitance and the resistance of the devices. This consequently causes a problem that the detection of the induction signal at the electrode by the signal detection circuit cannot be executed correctly, failing to execute correct position detection.
In the touch panel disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. SHO 60-123927, the width of the upper electrode is made narrower than the width of the lower electrode so as to prevent the upper electrode from functioning as an electrostatic shield. Then, the problems resulting from such an electrostatic shield occurs even in a duty ratio type display-integrated type tablet device as shown in FIG. 35. However, the duty ratio type display-integrated type tablet device has the problem that the width of the upper electrode cannot be made narrower than the width of the lower electrode, since a pixel is formed in each area where the upper electrode and the lower electrode intersect with each other.